To help vehicle drivers lower their speed before reaching critical road passages like curves or congested areas, Curve Speed Warning (CSW) systems have been developed. Such systems alert the driver by providing warnings (audible, visible, haptic, etc.) to make the driver aware that the vehicle speed may be too high for a safe and/or comfortable negotiation of the upcoming curve. Such systems may, for example, compare the predicted or projected speed of the vehicle with a predetermined maximum limit when it approaches a known or expected traffic environment (an area of congested traffic, for example) requiring low speed in order to be managed safety. If the speed is above the limit, the system warns the driver. Also known are so-called Curve Speed Control (CSC) systems which autonomously lower the speed of a vehicle before a curve or any other known traffic environment requiring lower speed.
U.S. Pat. No. 7,400,963 B2 discloses a vehicle curve speed control system that includes a map database representing a current vehicle path and a locator device communicatively coupled to the database and configured to determine the location of the vehicle on the path. The system further includes a controller configured to identify approaching curve points of a curve in terms of curvature or radius, and determine a desired speed profile based on driver preference and/or vehicle characteristic input. An acceleration profile is determined, based on the current vehicle speed, and desired speed profile. An acceleration or deceleration command at the present control loop is modified towards achieving an optimal curve speed and is delivered to either a brake or an acceleration module to automatically accelerate or decelerate the vehicle accordingly.
A natural limit for a vehicle's acceleration and deceleration is established by the friction available between the vehicle's tires and surface of the road on which it is travelling. Systems as described above take into account a maximum possible acceleration in either the longitudinal direction (which may be caused by braking or adding power) or the lateral direction (which may be caused by centripetal force), but fail to take a combination thereof into account. If, for example, a driver brakes at the same time as steering through a small-radius curve, the lateral acceleration limit may not be reached, but due to the longitudinal acceleration caused by braking the combined lateral/longitudinal limit may be exceeded, with the result that the tires may lose grip with the road.